1. Field of the Invention
The present invention relates to a method for controlling a switch-type reluctance motor and its apparatus and, in particular, relates to counter electromotive force compensation for a switch-type reluctance motor.
2. Description of the Related Art
The switch-type reluctance motor is designed to sequentially change by switching the coils to be excited when applied to the switch-type reluctance motor (e.g., a variable reluctance motor), which comprises a rotor and a stator respectively having salient poles and generates torque caused by magnetic attraction occurring in the salient poles when electric current is supplied to the coils wound round the salient poles of the stator.
FIGS. 10(a)-10(b) illustrate the method by which a reluctance motor is rotated. If the coil in phase A is excited when the rotor and the stator have the positional relationship shown in FIG. 10(a), the rotor starts to rotate in the counter-clockwise direction. If the coil in phase B is excited when the rotor and the stator have the positional relationship shown in FIG. 10(b), the rotor starts to rotate in the counter-clockwise direction. Likewise, if the coil in phase C is excited when the rotor and the stator have the positional relationship shown in FIG. 10(c), the rotor starts to rotate in the counter-clockwise direction.
In contrast, if the coil in phase B is excited in the state shown in FIG. 10(a), the coil in phase C in the state shown in FIG. 10(b) and the coil in phase A in the state shown in FIG. 10(c), the rotor starts to rotate in the clockwise direction.
Consequently, the phase of the coil, in which an electric current is to be applied, is determined based upon the position of the rotor, i.e., the electrical angle of the rotor, regardless of the direction of the electrical current to be supplied to the stator coil.
If an electric current is supplied to the coils between the position at which the stator salient poles and the rotor salient poles start to face each other and the position at which they completely face each other, a torque is generated in the direction of the rotation of the rotor.
FIGS. 11(a)-11(b) illustrate the torque in a reluctance motor. For instance, as shown in FIG. 11(a), if an electric current is supplied to the coil in phase A (the coil wound around the salient pole 20A) from the position at which the salient pole 20A in phase A of the stator 20 and the salient pole 21a, one of the salient poles of the rotor 21, start to face each other, the salient pole 20A in phase A of the stator 20 attracts the rotor salient pole 21a, to generate a torque that rotates the rotor 21 in the counter-clockwise direction as in FIG. 11(a). If the electric current is supplied in phase A until the rotor reaches the position at which the salient pole 20A in phase A of the stator 20 and the rotor salient pole 21a completely face each other, as shown in FIG. 11(b), a counter-clockwise torque is generated.
However, if the electric current is supplied in phase A until the rotor reaches a position beyond the position of the rotor 21 shown in FIG. 11(b) by rotating in the counter-clockwise direction, a clockwise torque is generated. That is, the torque is always generated in the direction in which magnetic resistance is reduced.
Now, the rotor position at which the stator salient pole 20A in phase A and one of the salient poles of the rotor 21 do not face each other at all, as shown in FIG. 11(a), is set as the electrical angle 0.degree., the rotor position at which the stator salient pole 20A and the rotor salient pole completely become opposite to each other, as shown in FIG. 11(b) as the electrical angle 180.degree., and the rotor position at which the stator salient pole 20A and the rotor salient pole do not face each other at all again, as the electrical angle 380.degree.. With the electrical angles thus set, if an electric current is applied between the electrical angle 0.degree. and the electrical angle 180.degree., a counter-clockwise torque is generated. If an electric current is applied between the electrical angle 180.degree. and the electrical angle 360.degree., a clockwise torque is generated.
Development of torque depends upon the magnetic flux .PHI. generated between the stator and the rotor, and an electromotive force is generated in the motor due to a change in the flux with time. This electromotive force can be approximated with the following equation: EQU V=-d.PHI./d t =-(.differential..PHI./.differential.i.times..differential.i/.differential .t.times..differential..PHI./.differential..theta..times..differential..the ta./.differential.t) (1)
Where i is the electric current, and .theta. the electrical angle.
In Eq. (1) above, the second term, (-.differential..PHI./.differential..theta..times..differential..theta./.d ifferential.t) represents the counter electromotive force. During low speed rotation (when .differential..theta./.differential.y is small), since the effect of the term of this counter electromotive force term is small, compensation through PI control or PID control of the current suffices. However, during high speed rotation (when .differential..theta./.differential.t is large), the voltage due to the term of this counter electromotive force becomes large. Because of this, the voltage which is actually applied to a coil changes greatly in correspondence to the speed, and sufficient compensation cannot be performed through current PI control or PID control.
To deal with this, in a conventional switch-type reluctance motor, (.differential..PHI./.differential..theta.), the second term, is set as a constant value, and, by adding a command value which is in proportion to the speed, voltage drops and voltage increases due to the counter electromotive force are compensated, so that the effect of the counter electromotive force is compensated.
FIG. 12 is a block diagram illustrating counter electromotive force compensation in a switch-type reluctance motor in the prior art. The speed of the motor 5 can be obtained as a feedback speed via a rotary encoder 6 and a speed detector term 7. The value obtained by subtracting the aforementioned feedback speed from the speed command v cmd is input to a block 1 for PI compensation. The block 1 for PI compensation outputs an electric current command i cmd. The electric current that is fed back from a PWM amplifier 4 via a block 8 for current detection is subtracted from the electric current command i cmd. This difference is input to a block 2 for excitation phase switching via a block 11 for phase advance compensation.
A block 20 for counter electromotive force compensation outputs a counter electromotive force compensation value which corresponds to the speed output from the block 7 for speed detection. The counter electromotive force compensation value is added to the voltage command from a block 3 for current loop gain, and the sum is inputted to the PWM amplifier 4 to send a command to the motor 5.
The counter electromotive force compensation is performed in such a manner that, as shown in FIG. 13(a)-13(b) instance, the current value ifb, which is reduced due to the counter electromotive force, is corrected towards the value of the current command I cmd.
In a switch-type reluctance motor in the prior art, counter electromotive force compensation is performed in the following manner: .differential..PHI./.differential..theta., which is the change in the magnetic flux .PHI. relative to the change in the electrical angle .theta. in the counter electromotive force term, is set as a constant value K. With the counter electromotive force Vr expressed as Vr=K.times..differential..PHI./.differential..theta., a command value which is in proportion to the speed .differential..PHI./.differential..theta. is added to. In this manner, drops and rises of voltage due to the counter electromotive force are compensated.
However, in a reluctance motor, .differential..PHI./.differential..theta. is not a constant value, but is a value dependent on the electrical angle .theta. and the current i. Because of this, when .differential..PHI./.differential..theta. is set as a constant value, as it is in counter electromotive force compensation in the prior art, there is a problem such that compensation is either excessive or inadequate depending upon the conditions.
FIGS. 14(a)-14(b) illustrate the state of the counter electromotive force Vr when .differential..PHI./.differential..theta. depends Upon the electrical angle .theta.. FIG. 14(a) illustrates the case where the excitation is for acceleration, constant speed and deceleration, while FIG. 14(b) shows the magnetic flux .PHI., and FIG. 14(c) .differential..PHI./.differential..theta. and FIG. 14(d) show the counter electromotive force Vr. Note that FIG. 14 shows a case where the electric current i is constant. As shown in FIG. 14(d), the counter electromotive force Vr changes in correspondence to the electrical angle .theta.. When the speed is increasing, as well as when the speed is constant, a negative counter electromotive force Vr is generated, while, a positive counter electromotive force Vr is generated when the speed is decreasing.
FIGS. 15(a)-15(b) illustrate the change in the current value caused by the counter electromotive force during excitation for acceleration, constant speed and deceleration. As shown in FIG. 15, in each acceleration state, the current value changes due to a counter electromotive force in the direction indicated with the arrow to generate the current curve indicated with the dotted line. As shown in FIG. 15(a), during acceleration operation and constant speed operation, the current value is reduced, since the counter electromotive force Vr is a negative value, as can be seen in FIG. 14(d). Also, as shown in FIG. 15(b), during deceleration operation, the current value increases as shown in FIG. 14(d), since the counter electromotive force is a positive value.
Likewise, even when .differential..PHI./.differential..theta. is dependent on the electric current value i, the change in the counter electromotive force Vr causes the current value to change. Thus, like the case of conventional compensation against counter electromotive force, if the compensation against the counter electromotive force is applied uniformly without varying the compensation depending on electric angle .theta. and electric current i, it is not possible for the electric current to be compensated corresponding to the variation of the current as is shown in FIGS. 15(a)-15(b), thereby causing the compensation to be either excessive or insufficient.